Evolving wireless communications standards continue to demand extremely high performance from the antennas used in mobile handsets. Modern antennas are expected to be compact while maintaining a high quality factor and a broad operating bandwidth. Due to carrier aggregation applications, a single antenna may be required to simultaneously send and/or receive signals at five or more different bands. For example, in one carrier aggregation application, an antenna may be expected to simultaneously transmit a band 17 uplink signal, receive a band 17 downlink signal, receive a band 1 downlink signal, receive a global positioning system (GPS) signal, and send and receive WiFi signals. Generally, a standalone antenna cannot meet the demanding performance standards dictated by the wireless standards when transmitting and receiving multiple signals. Accordingly, antenna tuning circuitry is often coupled to an antenna in order to improve the performance of the antenna. Specifically, the antenna tuning circuitry is generally configured to selectively couple one or more impedances to a resonant conducting element in the antenna in order to alter the resonant frequency of the resonant conducting element. The antenna may thus be “tuned” to a specific frequency or group of frequencies, which may increase the performance of the antenna in certain operating conditions.
FIG. 1 shows conventional antenna tuning circuitry 10. The antenna tuning circuitry 10 is coupled to a resonant conducting element 12 of an antenna 14 via an antenna tuning node 16. The conventional antenna tuning circuitry 10 includes a first antenna tuning inductor LAT1, a second antenna tuning inductor LAT2, a third antenna tuning inductor LAT3, a first antenna tuning switch SWAT1, a second antenna tuning switch SWAT2, and a third antenna tuning switch SWAT3. The first antenna tuning inductor LAT1 is coupled in series with the first antenna tuning switch SWAT1 between the antenna tuning node 16 and ground. The second antenna tuning inductor LAT2 is coupled in series with the second antenna tuning switch SWAT2 between the antenna tuning node 16 and ground. Finally, the third antenna tuning inductor LAT3 is coupled in series with the third antenna tuning switch SWAT3 between the antenna tuning node 16 and ground. Control circuitry 18 is coupled to the first antenna tuning switch SWAT1, the second antenna tuning switch SWAT2, and the third antenna tuning switch SWAT3 in order to control the state of the antenna tuning switches SWAT.
In operation, the control circuitry 18 opens or closes the first antenna tuning switch SWAT1, the second antenna tuning switch SWAT2, and/or the third antenna tuning switch SWAT3, either separately or together, in order to alter the impedance of the resonant conducting element 12 of the antenna 14. Changing the impedance of the resonant conducting element 12 effectively changes the resonant frequency thereof, thereby “tuning” the antenna 14 to a desired frequency or frequencies. Accordingly, the antenna 14 may more easily transmit or receive signals about a desired frequency or frequencies.
Although effective at “tuning” the antenna 14, the switching components present in the conventional antenna tuning circuitry 10 may degrade the performance of the antenna 14 in certain operating conditions. Specifically, the first antenna tuning switch SWAT1, the second antenna tuning switch SWAT2, and/or the third antenna tuning switch SWAT3 may generate harmonic signals, which are subsequently delivered to receive circuitry attached to the antenna 14 and/or transmitted from the antenna 14. Because the harmonic signals may be generated in response to a high-power transmit signal, such harmonic signals may cause desensitization of the receive circuitry, particularly when operating in a carrier aggregation configuration in which a receive frequency band includes one or more harmonic frequencies of a transmit signal. For example, a carrier aggregation configuration in which signals are simultaneously transmitted and received about bands 4 and 17 may be problematic, as the third harmonic of the band 17 uplink frequency range (704-716 MHz) falls squarely within the band 4 downlink frequency range (2110-2155 MHz).
Accordingly, there is a need for antenna tuning circuitry capable of altering the impedance and thus the resonant frequency of one or more resonant conducting elements in an antenna, while simultaneously avoiding or mitigating the generation of harmonic interference by the antenna tuning circuitry.